The United States of America has a policy of utilizing the present national coal reserves of approximately 3.5 trillion tons of coal in the 820 power plants that will burn coal over the next 100 years. This will produce about 10.29 trillion tons of carbon dioxide to be vented to the atmosphere. The world is now being made aware of the "greenhouse effect" which predicts that the accumulation of carbon dioxide and other gases in the atmosphere will raise the global temperature by about 2.degree. C. by 2050 and by about 5.degree. C. by 2100. This is expected to be disastrous and accordingly, all nations are preparing immediately to reduce carbon dioxide emissions drastically. The United States Government has issued a report of a study of this problem "Can We Delay a Greenhouse Warning?" This study notes that control of CO.sub.2 emissions from plants is an important step to take, and that the only technically feasible process is that of absorbing CO.sub.2 in nonethanolamine (MEA process), but that process is too costly in that it seriously reduces the capacities of the plants so much that it would not be economically feasible. Applicant's process is intended to meet that problem head-on and provide an economically feasible process for removing CO.sub.2 from stack gases or other gas streams low in CO.sub.2 content, i.e., less than about 85% CO.sub.2.
Another advantage of the present process is to provide carbon dioxide and nitrogen for use in programs of enhanced oil recovery (EOR). These programs are designed to go beyond the present art of primary and secondary methods of recovering petroleum from underground reservoirs. Only about 25-30% of the petroleum is recovered by the conventional primary and secondary methods. EOR programs increase that recovery to about 45-50% using carbon dioxide and nitrogen. Approximately 900-5400 cubic meters of CO.sub.2 are required per cubic meter of petroleum recovered. The applicant's process will provide an economical source of CO.sub.2 for such a process.
The process of this invention was developed specifically, over a period of about 6 years to recover carbon dioxide economically from gas streams which all other processes, e.g., MEA process, fail to do at all or fail to do in an economically feasible fashion. Preferred procedures in the process of this invention are accomplished by using any of three types of patented separators for the separation and liquefaction of carbon dioxide from the treated gas stream contaminated with carbon dioxide. The patented separators are described in U.S. Pat. Nos. 4,498,303; 4,572,728; or 4,639,262; and they describe gas-to-gas separation and gas-to-liquid separation. The step of separation is the key to an economically feasible process that does not rely on any expensive solution step such as in the MEA process. Other features of the present process are to utilize for heating or cooling any of the various gas and liquid streams in the process for heat exchange with other streams in the process. Furthermore, heat energy is converted to kinetic energy by expanding pressurized gas in turbines that may drive electric generators to produce electric power for use in the plant. The treated gas streams contain substantial amounts of nitrogen and oxygen and these gases are separated, purified and liquefied to produce valuable products that may be sold commercially and thereby reduce overall costs of removing carbon dioxide from flue gases and other gas streams vented to the atmosphere. There is no teaching in the prior art to expect that a process of this type would be successful in an economic sense. As a matter of fact, the consensus of the industry was that it would be impossible to accomplish. Hence, the MEA process was considered by the U.S. Government to be the only way to separate carbon dioxide from low purity streams, i.e., less than about 85% CO.sub.2.
The present process also purifies the flue gas of oxides of sulfur and the oxides of nitrogen so that the purified gas stream that is vented to the atmosphere will meet more stringent specifications than the Government's Environmental Protection Agency standards. The contaminating flue gas stream will be purified of sulfur dioxide to less than 0.3 PPM, carbon monoxide less than 10 PPM, and oxides of nitrogen to less than 10 PPM by volume. These food grade carbon dioxide specifications are current industry standards. The purification of the contaminating gas of vaporous odors and particulates are not part of this invention and therefore, are not discussed for both simplicity and proprietary reasons.
Various present methods of liquefying high purity 90% or better carbon dixoide gas are well known. Typically, the liquefaction process of a relatively pure carbon dioxide comprises of compressing the gaseous carbon dioxide to a pressure of approximately 233.85 psig to 312.1 psig and then removing the latent heat of condensation with a secondary refrigerant at an evaporating temperature below the saturation temperature of the carbon dioxide pressure or -12.degree. F. or -4.degree. F. respectively. The theoretical range of pressures over which vaporous carbon dioxide can be condensed to a liquid is approximately 60.43 psig to 1057.4 psig.
Low purity carbon dioxide also contains contaminating gases with a lower temperature of condensation than carbon dioxide and these contaminating gases require a lower temperature refrigerant to condense than the carbon dioxide vapors. Therefore, the carbon dioxide may be separated from a contaminating gas source by fractional condensation. This invention specifically removes the carbon dioxide vapors from a gas stream between any compressor created saturation point down to the triple point of carbon dioxide. Any carbon dioxide below the triple point is unrecoverable.
This invention relates to a process for recovering carbon dioxide vapors from a gas stream such as flue gas, industrial waste gas streams or any other low purity carbon dioxide gas stream, particularly to a process for recovering carbon dioxide at purities of less than about 85% that are too low to recover economically by a conventional carbon dioxide liquefaction system. It specifically replaces the MEA chemical absorption process. This invention produces carbon dioxide liquid or vapor at a substantial utility cost reduction below all existing MEA technology.
It has proved to be most difficult and costly to recover, purify and liquefy the carbon dioxide vapors when they are present in low concentrations in a gas stream. Thus, all known processes which recover carbon dioxide vapors present in a gas at low concentrations involve high investment and/or production utility costs. In particular, in all MEA type absorption processes, the excessive amounts of steam required to regenerate the absorbent prohibits economic recovery of carbon dioxide from low purity gas sources, such as a steam boiler flue stack gases which are in the magnitude of 8 to 15% volume carbon dioxide purity.
There are basically three types of carbon dioxide vapor and gas stream recovery combinations: (1) 85-100% pure carbon dioxide vapor-laden streams, (2) less than 85% and greater than 50% carbon dioxide vapor-laden gas streams, (3) 50% and less carbon dioxide vapor-laden gas streams. In Item (2) above, we are removing the non-condensable gases from the condensable carbon dioxide vapors. In Item (3) above, we are removing the condensable carbon dioxide vapor from the non-condensable gases. The above is determined mathematically by the ratio of the carbon dioxide vapor pressure to the partial pressure of the non-condensable gas stream. When this ratio is greater than one, we are removing the non-condensable gas from the carbon dioxide vapor. When this ratio is equal to one or less, we are removing the carbon dioxide vapor from the non-condensable gas. When we are removing a non-condensable gas from a carbon dioxide vapor we reach the point in fractional condensation where this ratio becomes one and then the carbon dioxide vapor must be removed from the non-condensable gas.
The removal of carbon dioxide from the non-condensable gas can occur only when the carbon dioxide vapor pressure is above the carbon dioxide triple point of -69.9.degree. F. The removal of carbon dioxide vapor pressure below the triple point will cause freezing of the carbon dioxide. Therefore, the carbon dioxide vapors contained in the non-condensable gas who's dewpoint is below the triple point is non-recoverable vapors and are vented.
The invention has two types of non-condensable vent procedures; a continuous vent process and a batch vent process. The batch vent process is applicable for approximately 50% or greater carbon dioxide purity gas stream. It's primary advantage is that it minimizes the amount of non-recoverable carbon dioxide vapor vented. It operates on the basic principle that the higher the non-condensable gas pressure, the less carbon dioxide vapor at saturation conditions it will hold. The carbon dioxide vapor pressure maintained equilibrium conditions and any increase in carbon dioxide vapor pressure will condense to a liquid. The continuous vent process will vent all the carbon dioxide vapors in the non-condensable gas stream. Example: a 95% carbon dioxide vapor stream at -12.degree. F. will vent 5.3% of the carbon dioxide vapor on a continuous vent process. The same 95% carbon dioxide vapor stream will vent 1.0% of the carbon dioxide vapor on a batch vent process.